1. Field of the Invention
Embodiments of the subject matter disclosed herein generally relate to methods and devices that relocate a surge margin after occurrence of a surge event is detected based on pattern recognition in an evolution of a discharge pressure.
2. Description of Related Art
Centrifugal compressors are a class of radial-flow work-absorbing turbomachinery. In a centrifugal compressor, the pressure is increased by adding kinetic-energy/velocity to a continuous flow of fluid through rotation of a rotor or an impeller of the compressor. Centrifugal compressors are frequently used in pipeline transport of natural gas to move the gas from a production site to consumers, in oil refineries, refrigeration systems, gas turbines, etc.
Centrifugal compressor's operation may be affected by the occurrence of a surge. Pressure of a flow of fluid passing through the compressor increases from a surge pressure at the input of the compressor, to a discharge pressure at the output of the compressor. A surge phenomenon occurs when the compressor cannot add enough energy to overcome the system resistance, which results in a rapid flow and discharge pressure decrease. The surge may be accompanied by high vibrations, temperature increases and rapid changes in the axial thrust. These effects may damage the compressor. Most systems including compressors are designed to withstand occasional surging. However, repeated and long lasting surges may result in catastrophic failures.
The system operation during a surge event is unstable. Therefore, engineers try to operate compressors away from the compressor's stability limit, by adjusting a ratio of the pressures of fluid input into and discharged from the compressor, the fluid flow or other parameters that may be controlled. A surge margin provides a measure of how close an operating state of the compressor is to a surge state. Various parameters may be used for evaluating the surge margin. For example, a surge margin may be a ratio of a fluid flow input into the compressor which engineers consider safe (i.e., no surge is expected to occur) and a surge fluid flow at which a surge is likely to occur, all other operating conditions (e.g., a ratio of a surge pressure and a discharge pressure) except the fluid flow being the same.
FIG. 1 represents a diagram of a conventional system 1 including an expander 10 and a compressor 20. The conventional system 1 includes an anti-surge flow recirculation loop 30 providing a flow path from an output 32 of the compressor 20 to an input 34 of the compressor 20. Along the anti-surge flow recirculation loop 30 are located a surge detector 40 and an anti-surge valve 50. The anti-surge flow recirculation loop 30 may also include a gas cooler 60 and a flow element 70.
Depending on the operating states of the anti-surge valve 50, a gas flow may be recycled from the output 32 of the compressor 20 to the input 34 of the compressor 20. When the detector detects a surge trend, the anti-surge valve 50 is operated to break the surge cycle by adjusting the flow to reverse the surge trend. Conventionally, the anti-surge control and surge detection are independent. The conventional surge detection may only trip the system.
A surge shot is an event characterized by the occurrence of a surge trend. Due to potentially catastrophic effects of a surge event, it is desirable to operate the system with a sufficient surge margin to avoid occurrence of any surge event.
The surge detector 40 may detect an occurrence of a surge trend by monitoring a discharge pressure (pd) at the output 32 of the compressor 20. Conventionally, a surge trend is detected when the discharge pressure decreases rapidly (i.e., based on a first order derivative relative to time of the discharge pressure). A first order derivative of the discharge pressure is calculated mechanically in the surge detector 40 in FIG. 1, but it may alternatively be obtained electronically based on signal processing in an electronic surge detector described below relative to FIG. 2.
FIG. 2 illustrates a block diagram of a conventional electronic surge detector 100. The discharge pressure (V) is input to the calculation block 110 and to the add/subtract block 120. A time constant (T) is also input to the calculation block 110. The calculation block 110 outputs a value proportional to the discharged pressure (V) obtained using a first order lag filter with time constant T.
The add/subtract block 120 subtracts the discharge pressure from the value output by block 110, and outputs a value (A) that (expressed in Laplace transform nomenclature) is equal to—pdTs/(1+Ts), to the comparison block 130. The comparison block 130 sends a signal to the event counter block 140 if the value (A) received from block 120 is larger than a predetermined value (B), which is separately input to the comparison block 130.
The event counter 140 keeps track of a number of signals, which represent surge shots, received from the comparison block 130 within a predetermined time interval (T3surge), whose value is entered separately to the event counter 140. If two or more surge shots occur during a period equal to the predetermined time interval (T3surge), the event counter 140 outputs an alarm signal. If three or more surge shots occur during a period equal to the predetermined time interval (T3surge), the event counter 140 outputs a trip signal, signaling imminent trip (i.e., shut down) of the system.
The conventional surge detection has the disadvantage that a surge shot detection depends only on an instantaneous discharge pressure slope (i.e., the first derivative of the discharge pressure). However, a discharge pressure versus time pattern typically occurring in after the surge trend has more complex features. For example, after the discharge pressure drops abruptly in a relatively short time a minimum pressure value is reached, and then the discharge pressure increases again. Conventional recognition of this surge pattern is weak because it considers only on the first time derivative of the discharge pressure at the beginning of the surge shot.
Additionally, the conventional system provides no recovery action if the anti-surge controller operates based on an erroneously configured surge line, the only response of the conventional system being tripping of the system. For example, if the margin is set too low with respect to the real surge line, the anti-surge control through the loop 30 cannot maintain a minimum safe flow through compressor and a surge trend cycle may occur at a frequency that depends also on a closure rate of the anti-surge valve 50.
Another disadvantage of the conventional system 1 is that an amplification applied to the time derivative of the discharge pressure is related to the predetermined threshold used for determining the occurrence of a surge shot.
Accordingly, it would be desirable to provide systems and methods that avoid the afore-described problems and drawbacks.